1. Field of the Invention
The present invention relates to a fluid machine, which is operable in a pump mode for pressurizing and discharging fluid and is also operable in a motor mode for converting fluid pressure exerted at the time of expansion of fluid into kinetic energy and thereafter outputting the kinetic energy as mechanical energy, and also relates to a waste heat recovering system having the same.
2. Description of Related Art
In a previously proposed vapor compression refrigeration system, which has a Rankine cycle, when energy is recovered through the Rankine cycle, a compressor of the vapor compression refrigeration system is used as an expander, as disclosed in, for example, Japanese Patent No. 2540738.
In the compressor, gas, such as gas phase refrigerant, is drawn into a working chamber upon input of external mechanical energy to the compressor and is compressed by reducing a volume of the working chamber, and thereafter the compressed gas is discharged from the compressor. In the expander, high pressure gas is introduced into the working chamber, and the working chamber is expanded by the pressure of the high pressure gas to output, for example, mechanical energy. Thus, when the compressor is used as the expander, a flow direction of refrigerant needs to be reversed.
However, in the invention recited in Japanese Patent No. 2540738, a refrigerant inlet and a refrigerant outlet of the expander (compressor) used at the time of recovering energy are the same as a refrigerant inlet and a refrigerant outlet of the compressor (expander) used at the time of exerting refrigeration capacity in the vapor compression refrigeration system. Thus, the single compressor cannot be used as the expander. As a result, one of the Rankine cycle and the vapor compression refrigeration system cannot be operated properly.
More specifically, in the compressor, the gas is compressed by reducing the volume of the working chamber upon displacement, i.e., movement of a movable member, such as a piston or a movable scroll. Thus, a check valve is provided in a discharge port, which communicates between the working chamber and a high pressure chamber (discharge chamber), to prevent backflow of the gas from the high pressure chamber to the working chamber.
On the other hand, in the expander, the high pressure gas is conducted from the high pressure chamber to the working chamber to displace, i.e., to drive the movable member and thereby to obtain mechanical output. Thus, even in a case where a means for reversing the gas inlet and the gas outlet is provided in the above arrangement, when the compressor is operated as the expander, the high pressure gas cannot be supplied to the working chamber due to the presence of the check valve, which blocks flow of the high pressure gas into the working chamber. As a result, the compressor cannot be used as the expander even if the means for reversing the gas inlet and the gas outlet is provided.
Furthermore, a volume, i.e., a capacitance of the compressor is normally determined at the time of operation of the vapor compression refrigeration system. However, the pressure exerted in the compressor at the time of operation of the Rankine cycle is higher than the pressure exerted in the compressor at the time of operation of the vapor compression refrigeration system. Thus, even when the same weight flow rate of gas is used in both the time of operation of the vapor compression refrigeration system and the time of operation of the Rankine cycle, a volume flow rate of the Rankine cycle is reduced in comparison to a volume flow rate of the vapor compression refrigeration system. As a result, a rotational speed of the compressor used as the expander is reduced, and a degree of leakage of fluid per rotation of the compressor is increased to decrease an efficiency of the expander.